The invention relates to a metal halide lamp provided with a discharge vessel with a ceramic wall which encloses a discharge space in which an electrode is arranged, which discharge vessel space is sealed off by means of a ceramic plug in which a lead-through element is fastened in a gastight manner by means of a sealing ceramic, said lead-through element serving to form an electrical connection between the electrode and a conductor outside the discharge vessel.
The term xe2x80x9cceramic wallxe2x80x9d in the present description and claims is understood to mean a wall of metal oxide such as, for example, sapphire or densely sintered polycrystalline Al2O3 as well as of metal nitride, for example AIN.
A lamp of the kind mentioned in the opening paragraph is known from EP 0587238 =U.S. Pat. No. 5,424,609 (N14191). The lead-through element in the known lamp is built up from at least two electrically conducting parts. The lead-through element consists of an Nb rod at the area of the ceramic seal. The advantage of the use of Nb is that it is highly ductile on the one hand, while on the other hand it has a coefficient of expansion which differs only very slightly from that of the ceramic material used for the discharge vessel. A disadvantage is, however, that Nb is not resistant to halides. This implies that the lead-through element in the known lamp comprises at least a second part which can be exposed to halides during a longer period, and that the Nb must be fully screened off from the discharge space, for example in that it is coated with the sealing ceramic. A further complicating factor is that the second part which is allowed to be exposed to halides will have a coefficient of expansion which differs considerably from that of the ceramic wall material. Besides the disadvantages described above relating to the complicated construction of the known lamp, a further disadvantage is found to occur in practice in the form of an attack on the sealing ceramic by the halide present, so that after some time the Nb comes into direct contact with the halides after all and the lamp fails prematurely.
GB 1435244 discloses a lamp in which a foil is sintered as a lead-through conductor between an end of a ceramic wall of a discharge lamp and a ceramic closing disc. Although a lead-through construction highly resistant to halides is possible per se in this manner, it is necessary for such a construction to be completed during the manufacture of the lamp vessel, i.e. preceding the actual lamp manufacture during which the filling is added to the discharge vessel. This accordingly leads to a more complicated lamp manufacture, which is highly undesirable in general in a modem mass production process.
A lamp is known from U.S. Pat. No. 42,777,15 in which a closed coiled foil extends as a lead-through conductor through an end plug of a discharge vessel from the interior to outside the discharge vessel, connected thereto by means of melting glass. The foil is provided around a ceramic rod. A disadvantage of such a coiled foil is that it is found to be practically impossible to keep the foil correctly positioned during lamp manufacture, so that a non-hermetic seal is obtained at the area of the lead-through conductor.
The invention has for its object to provide a lamp of the kind mentioned in the opening paragraph which is of a simple construction and in which the risk of a premature failure owing to halide attacks is counteracted.
According to the invention, a lamp of the kind mentioned in the opening paragraph is for this purpose characterized in that the lead-through element comprises a ceramic core which is connected in a gastight manner to the ceramic plug by means of sealing ceramic in a direct joint and which is provided with metal envelopes on either side of the direct joint, which metal envelopes are interconnected by means of a strip-shaped connecting part.
An advantage of the lamp according to the invention is that the gastight closure with sealing ceramic is achieved as a direct joint between the ceramic wall and the ceramic core of the lead-through element. It is surprisingly found that the strip-shaped connecting part is entirely enveloped by the sealing ceramic while achieving a permanent adhesion and has no appreciable influence on the permanence of the lead-through construction, and thus on lamp life. A problem relating to the difference in coefficient of expansion is avoided in this manner. In a further improvement of the lamp according to the invention, the strip-shaped connecting part is provided with knife edges. This is found to promote strongly a good and permanent adhesion between the sealing ceramic and the stripshaped connecting part. In an advantageous embodiment of the lamp according to the invention, the metal envelope is fastened to the ceramic core by means of sealing ceramic outside the discharge vessel. This has the advantage that the gastight sealing ceramic closure of the discharge vessel and an adhesive joint between the ceramic core and the metal envelope can be realized in a single operational step.
In a further advantageous embodiment of the lamp, the metal envelopes are interconnected on either side of the direct joint by means of two strip-shaped connecting parts. One of the advantages of this is an improved stiffness of the assembly of the metal parts which form the envelopes and connecting strips in the finished lamp. This is of major importance for a fast and reliable mass production of the lamp. Preferably, the two strip-shaped connecting parts are positioned diametrically opposite one another. This achieves both an optimum stiffness and an equally divided load on the lead-through in the operational state of the lamp.
Preferably, the metal envelopes and their strip-shaped connecting parts are made of Mo, because this was found to be suitable as an electrical conductor as well as highly resistant to halides. In a preferred embodiment, the strip-shaped connecting parts have a joint width B of at least 0.25 O and at most 0.34 O, O being the circumference of one of the envelopes. Such a width is found to be an optimum for realizing on the one hand a good current conduction and a good stiffness of the assembly of the metal parts which form the envelopes and connecting strips in the finished lamp, and on the other hand is still small enough not to detract from the permanence of the gastight sealing of the lead-through construction. Preferably, the thickness of the envelopes and the strip-shaped connecting parts lies between 10 xcexcm and 200 xcexcm. Given such a choice of thickness, it was found to be possible to manufacture the entire assembly of envelopes and strip-shaped connecting parts from a pipe- or tube-shaped material, whereupon this can be passed over the ceramic core in a simple manner. A greater thickness increases the risk that the permanence of the lead-through construction becomes less reliable owing to the difference in coefficient of expansion.
For realizing a good useful life of the gastight seal, it is desirable for the sealing ceramic to extend over a length of a few mm, preferably at least 3 mm, inside the ceramic plug. It is favorable when the sealing ceramic extends to over the metal envelope present in the ceramic plug, in particular if the lamp has small dimensions.
An additional advantage of the lamp according to the invention is that the use of Nb is not necessary for current conduction outside the discharge vessel. This offers the possibility of operating the discharge vessel in the air.
The metal envelopes on either side of the direct joint may have mutually different lengths. It is favorable, however, for reasons of production efficiency if the metal envelopes have the same length.